In the adaptive immune response, foreign antigens are recognized by receptor molecules on B lymphocytes (e.g., immunoglobulins) and T lymphocytes (e.g., T cell receptor or TCR). These foreign antigens are presented on the surface of cells as peptide fragments by specialized proteins, generically referred to as major histocompatibility complex (MHC) molecules. MHC molecules are encoded by multiple loci that are found as a linked cluster of genes that spans about 4 Mb. In mice, the MHC genes are found on chromosome 17, and for historical reasons are referred to as the histocompatibility 2 (H-2) genes. In humans, the genes are found on chromosome 6 and are called human leukocyte antigen (HLA) genes. The loci in mice and humans are polygenic; they include three highly polymorphic classes of MHC genes (class I, II and III) that exhibit similar organization in human and murine genomes (see FIG. 2 and FIG. 3, respectively).
MHC loci exhibit the highest polymorphism in the genome; some genes are represented by >300 alleles (e.g., human HLA-DRβ and human HLA-B). All class I and II MHC genes can present peptide fragments, but each gene expresses a protein with different binding characteristics, reflecting polymorphisms and allelic variants. Any given individual has a unique range of peptide fragments that can be presented on the cell surface to B and T cells in the course of an immune response.
Both humans and mice have class II MHC genes (see FIGS. 2 and 3). In humans, the classical MHC II genes are termed HLA-DP, HLA-DQ, and HLA-DR, whereas in mice they are H-2A and H-2E (often abbreviated as I-A and I-E, respectively). Additional proteins encoded by genes in the MHC II locus, HLA-DM and HLA-DO in humans, and H-2M and H-2O in mice, are not found on the cell surface, but reside in the endocytic compartment and ensure proper loading of MHC II molecules with peptides. Class II molecules consist of two polypeptide chains: α chain and β chain. The extracellular portion of the α chain contains two extracellular domains, α1 and α2; and the extracellular portion of the β chain also contains two extracellular domains, β1 and β2 (see FIG. 1). The α and the β chains are non-covalently associated with each other.
MHC class II molecules are expressed on antigen-presenting cells (APCs), e.g., B cells, macrophages, dendritic cells, endothelial cells during a course of inflammation, etc. MHC II molecules expressed on the surface of APCs typically present antigens generated in intracellular vesicles to CD4+ T cells. In order to participate in CD4+ T cell engagement, the MHC class II complex with the antigen of interest must be sufficiently stable to survive long enough to engage a CD4+ T cell. When a CD4+ T helper cell is engaged by a foreign peptide/MHC II complex on the surface of APC, the T cell is activated to release cytokines that assist in immune response to the invader.
Not all antigens will provoke T cell activation due to tolerance mechanisms. However, in some diseases (e.g., cancer, autoimmune diseases) peptides derived from self-proteins become the target of the cellular component of the immune system, which results in destruction of cells presenting such peptides. There has been significant advancement in recognizing antigens that are clinically significant (e.g., antigens associated with various types of cancer). However, in order to improve identification and selection of peptides that will provoke a suitable response in a human T cell, in particular for peptides of clinically significant antigens, there remains a need for in vivo and in vitro systems that mimic aspects of human immune system. Thus, there is a need for biological systems (e.g., genetically modified non-human animals and cells) that can display components of a human immune system.